What Is #28: An Open Cluster?

Last week in this on-going "What Is" series, I took a look at globular clusters, which foreshadowed into open clusters. If globular clusters are old, up to a million star clusters, what are open clusters? In many ways, open clusters are the exact opposite of globular clusters, although they share the characteristic of being filled with stars that are all gravitationally bound to each other.

Pleiades by Juan lacruz 

Open clusters are star clusters that are typically very young at a few hundred million years or less. As a result they contain larger, brighter, and bluer stars and often still contain gas and dust. The image above is a great example of an open cluster. The Pleiades has several very bright, blueish stars and is visible to the naked eye.

Open clusters are typically smaller than some globular clusters but also contain far fewer stars at only a 1000 or so. If you are a star in this cluster, your night sky likely contains many of these bright blue stars. It's very possible there will be more stars in your night sky in an open cluster than a globular cluster given the old age of the dim stars in a globular cluster.

Since open clusters are young, they did not form in the halo of our galaxy, which lacks the gas to form new stars. Instead, these clusters formed in the disk of our galaxy, where the Sun resides, because there is still plenty of gas and dust to form new stars. The Sun, however, is not part of an open cluster. The Sun sits by itself, outside of any clusters.